The nucleic acid sequences of the RNA genome of mouse mammary tumor virus (MMTVs) for C3H and RIII mice that are transmitted via a non-germ line mechanism have been isolated. They were shown to be present as a provirus in the DNA of mammary tumors that arise early in life, but not in normal tissue or late mammary tumors of C3H and RIII mice. These viral nucleic acid sequences were also absent in the DNA from mammary tumors of low and moderate incidence mouse strains. The distribution of these nucleic acid sequences in the organs of normal and tumor bearing mice have been examined. By relaxing the conditions of hybridization, nucleic acid sequences related to, but not identical to, the RNA genome of MMTV have been detected in the DNAs of a variety of Mus species and in laboratory rats. Molecular hybridization studies with Mason-Pfizer virus (MPV) 70S RNA demonstrated that approximately 20% of the MPV genome is endogenous to rhesus monkeys and these sequences are highly conserved in other Old World monkeys by relaxing the hybridization conditions thus demonstrating the wide distribution of type-D retroviral sequences in primates.